1. Field of the Invention
Embodiments of the present invention relate generally to fluorescent materials containing rare earth elements and, more particularly, to such luminescent materials for exciting ultraviolet as well as visible light containing lead- and/or copper-containing compounds.
2. Description of the Related Art
Lead and copper activated materials are known for short wave excitation, e.g. from a low pressure mercury lamp, such as barium disilicate activated by lead (Keith H. Butler, The Pennsylvania State University Press, 1980, S 175, orthosilicate activated by lead (Keith H. Butler, The Pennsylvania State University Press, 1980, S. 181), akermanites activated by lead, or Ca-metasilicate activated by Pb2+.
Generally, the maxima of the emission bands of such lead activated phosphors are located between 290 nm and 370 nm at 254 nm excitation. Bariumdisilicate activated by lead is an U.V. emitting phosphor which currently is used in sun parlor lamps.
Lead has in the ground state 1S0 two outer electrons. The electron configuration of the ground state is d10s2, so that the lowest excited state has d10sp configuration. The excited sp configuration has four levels, 3P0, 3P1, 3P2 and 1P1, which can be achieved between 165.57 nm (3P0) and 104.88 nm (1P1) in the free ion. Transitions between 1S0 and 1P1 excited level are allowed by all selection rules. While transitions between 1S0 and 3P0 are only allowed with the lowest symmetry, transitions between 1S0 and 3P1 as well as 3P2 are allowed only under certain conditions. However, excitation between 180 and 370 nm has the same emission. Excitation with wavelength longer than 370 nm is not possible.
Otherwise, luminescent materials are known having lead as a host lattice component. Molybdate phosphors containing MoO42− centers are described in Bernhardt, H. J., Phys. Stat. Sol. (a), 91, 643, 1985. PbMoO4 shows at room temperature red emission with an emission maximum at 620 nm under photoexcitation at 360 nm.
However, such emission is not caused by lead itself. In molybdates the luminescence properties are not caused by the metal ion M2+ (M2+MoO4 where M2+=Ca, Sr, Cd, Zn, Ba, Pb etc). Here, defect centers of MoO42− ions coupled to O2−-ion vacancies seem to be the reason. Nevertheless, the Pb2+-ion influences the preferred emission properties because it stabilizes the host lattice.
As a familiar example, tungstates (Ca,Pb)WO4 as mixed crystals have a strong green emission with high quantum output of 75% (Blasse, G, Radiationless processes in luminescent materials, in Radiationless Processes, DiBartolo, B., Ed. Plenum Press, New York, 1980, 287). Under 250 nm excitation PbWO4 shows blue emission and under 313 nm excitation PbWO4 has an orange emission band, which can be caused by Schottky defects or by impurity ions (Phosphor Handbook, edited under the Auspice of Phosphor Research Society, CRC Press New York, 1998, S 205).
Copper was used as a monovalent activator in orthophosphates (Wanmaker, W. L. and Bakker, C., J. Electrochem. Soc., 106, 1027, 1959) with an emission maximum at 490 nm. The ground state of monovalent copper is a filled shell 3d10. That is the level 1S0. After exciting the lowest excited configuration is 3d94s. This configuration has two terms, 3D and 1D. The next higher configuration, 3d94p, gives 6 terms 3P°, 3F°, 3D°, 1F°, 1D° and 1P°. The transitions between the ground state 1S0 and the 1D or 3D are forbidden by parity or spin, respectively. In copper ions, the excitation to the crystal field levels of 4p terms are allowed. Emission will be got either by a direct return from the crystal field odd state to the ground state or by a combination of transitions first from the odd state to a crystal field level and after that a second transition from these 3D or 1D state of the 3d94s configuration to the ground state.
The ground state of bivalent copper has 3d9-configuration. That is the level 2D5/2. In the bivalent copper, one of the d-electrons can be excited to the 4s or 4p orbital. The lowest exciting configuration is the 3d84s with two quartet terms 4F, 4P and four doublet terms, 2F, 2D, 2P and 2G without emission caused by forbidden transitions. The higher exciting configuration is the 3d84p-configuration with four terms 4D°, 4G°, 4F°, and 4P°, where emission can occur.
Copper activated or co-activated sulphide-phosphors are well known and they are commercial used for cathode ray tubes. The green-emitting ZnS:Cu, Al (wherein, the copper is used as activator and Al is used as co-activator) is very important in CRT applications.
In zinc-sulphide phosphors, the luminescent materials can be classified into five kinds, depending on the relative ratio of the concentration of activators and co-activators (van Gool, W., Philips Res. Rept. Suppl., 3, 1, 1961). Here, the luminescent centers are formed from deep donors or deep acceptors, or by their association at the nearest-neighbor sites (Phosphor Handbook, edited under the Auspice of Phosphor Research Society, CRC Press New York, 1998, S. 238).
Orthophosphates activated by copper (Wanmaker, W. L., and Spier, H. L., JECS 109 (1962), 109), and pyrophosphates, alumosilicates, silicates, and tripolyphosphates all activated by copper are described in “Keith H. Butler, The Pennsylvania State University Press, 1980, S. 281”. However, such phosphors can only be used for a short wave U.V. excitation. Because of their unstable chemical properties and their temperature behavior, they cannot be used in fluorescent lamps.
It has been observed that conventional luminescent materials are generally unstable in water, air humidity, water steam and polar solvents.